It is known in the art relating to magnetic sensors to provide a sensor that determines the angular rotation of a ferromagnetic object by magnetically sensing changes in a magnetic field caused by movement of the ferromagnetic object through the magnetic field.
One application of such a sensor is used to determine the speed or angular position of an engine crankshaft for ignition timing. A wheel of the crankshaft has teeth spaced around its circumference that project toward the sensing device. As the crankshaft rotates, each tooth passes by the sensor and the strength of the sensor's magnetic field increases in the spatial area of the sensor. Subsequently, the strength of the magnetic field decreases as the tooth moves away from the sensor. The changes in the magnetic field can be used to determine the speed or angular position of the crankshaft.
Several types of sensors may be used to detect the changes in a magnetic field. One such device is called a Hall effect sensor; another is called a magnetoresistive sensor. A Hall effect sensor may have two magnetic sensing elements positioned adjacent a target wheel which has teeth and slots along its periphery. The voltage across the sensing elements changes with the passage of a tooth or a slot. The sensing elements generate two identical signals phase shifted by an angle corresponding to their spacing. One of the signals is subtracted from the other. The resultant differential signal is compared with a fixed threshold level in order to generate a digital signal corresponding to the teeth and slots of the target wheel.
The magnetic field strength of the sensor decreases in air as a function of the squared distance from the source. Accordingly, as an air gap between the sensor and the target wheel increases, the tooth/slot output voltage of the sensor decreases. When the sensor operates over a range of air gaps, the threshold level must be set low enough to intercept the lowest signal which occurs at the highest air gap and operating temperature. At smaller air gaps and/or lower temperatures the signal magnitude is larger and it crosses the fixed threshold level at a different point. This results in the sensor's dependence upon temperature and the air gap which both affect the ability to get an accurate reading.
A magnetoresistive sensor is a device whose resistance varies with the strength of a magnetic field applied to the device. The magnetoresistor is mounted within and perpendicular to a magnetic flux developed between a biasing magnet and a target wheel which has teeth and slots along its periphery. When the target wheel rotates, it moves linearly toward and away from the magnetoresistor. In doing so, it changes the reluctance of the magnetic circuit involving the sensor. This causes the magnetic flux through the magnetoresistor to vary in a manner corresponding to the position of the teeth and/or the slots of the target wheel. With the change in magnetic flux, there occurs a corresponding change in the magnetic field strength to which the sensor is exposed. A higher magnetic field strength increases the resistance and a lower magnetic field strength decreases the resistance. The changes in the magnetic field can be used to determine the angular position of the target wheel.
Such a device is disclosed in U.S. Pat. No. 5,754,042, assigned to the assignee of the present invention and issued May 19, 1998. The magnetoresistive sensor includes a biasing magnet and two magnetoresistors. The two magnetoresistors are mounted on a biasing magnet which is positioned adjacent a target wheel having teeth and slots along its periphery. The distance between the two magnetoresistors relative to teeth on the target wheel is such that their analog signals are phase shifted by 180.degree., resulting in time overlapping signals which intersect at precise angular positions of the target wheel. The intersection points can be used to determine the angular positions of the target wheel. By eliminating the use of a reference voltage to determine such angular positions, the accuracy of the sensor is independent of temperature and changes in air gap between the sensor and the target wheel. To achieve the desired phase shift, one sensor must detect a leading edge of a tooth while the other sensor detects a trailing edge of a tooth. Determination of the precise length of the slots is also required to produce the desired magnetoresistor output signals. The tooth/slot configuration of the target wheel is directly related to the positioning of the magnetoresistors.